1. Field of the Present Invention
The present invention generally relates to the field of electronic circuits and more particularly to digital flip-flop circuits.
2. History of Related Art
Flip-flop circuits are well known in the field of digital electronics. Referring to FIG. 1, a typical flip-flop circuit 100 is depicted. Flip-flop circuit 100 is configured to receive a clock input (C) on a clock input node 122 and a data input (D) on a data input node 123. Circuit 100 is further configured to produce a pair of complementary digital output signals comprising an output signal (Q) on an output node 126 and its logical complement output signal (QB) on output node 128.
The operation of circuit 100 will be described with reference to FIGS. 1 through 4 where FIGS. 2, 3, and 4 are equivalent circuit representations of circuit 100 under various states and where xe2x80x9conxe2x80x9d transistors are replaced with a source-to-drain short and xe2x80x9coffxe2x80x9d transistors are replaced with a source-to-drain open. As depicted in FIG. 1, circuit 100 includes n-channel transistors 101 through 106, p-channel transistors 107 through 110, and inverters 112, 114, 116, and 118. The p-channel transistors 107 through 109 are connected between Vdd and a control node 120 and their gate terminals are controlled by clock signal C, data signal D, and signal CBD respectively. The n-channel transistors 101 through 103 form a series connection between control node 120 and Vss or ground while their gate terminals are also controlled by signals C, D, and CBD respectively. The n-channel transistors 104, 105, and 106 form a series connection between output node 126 and Vss with their gates controlled by clock signal C, a control node 120, and signal CBD respectively.
When clock signal C is low (steady state) as represented in FIG. 2, p-channel transistor 107 turns on and pulls control node 120 high. When clock signal C subsequently transitions from low to high (FIG. 3), the state of control node 120 is determined by the state of input signal D for a duration or interval referred to herein as the clocking duration or clocking interval. The length of the clocking interval is determined by the series combination of inverters 112 through 116. The low to high transition of signal C ripples through inverters 112 through 116 to produce on node 124 a high-to-low transition on a signal identified as CBD (C Bar Delayed), which is a time-delayed complement of clock signal C. When the transition of clock signal C ripples through to node 124, CBD transitions low. When CBD is low (FIG. 4), p-channel transistor turns on 109 and pulls control node 120 high. Thus, the series inverters 112 through 116 produce a clocking interval window during which C and CBD are both high following a low-to-high transition of clock signal C.
Returning to FIG. 3, which represents the state of circuit 100 during the clocking interval, if D is low, p-channel transistor 108 will turn on thereby maintaining control node 120 in its high state. If D is high, p-channel transistors 107, 108, and 109 are cut off and n-channel transistors 101 through 103 are turned on. In this state, control node 120 will be pulled low. Because, however, control node 120 is connected to the capacitive gate terminals of two transistors (105 and 110) and to the series resistance of transistors 101 through 103, there will be an inherent delay associated with the high to low transition of control node 120. Moreover, because control node 120 is connected to the gate electrode of the output transistor pair (transistor 105 and transistor 110), any delay in the transition of control node 120 is propagated to output nodes 126 and 128 thereby negatively impacting performance. In addition, it will be apparent that the rise time and fall time associated with the depicted circuit configuration are unequal or asymmetrical because of the relatively long time it takes control node 120 to transition low. Asymmetrical timing in digital circuits is typically undesirable because timing requirements are typically understood to be independent of the data. It would be desirable, therefore, to implement a flip-flop circuit that achieved substantially symmetrical performance at low power and without a significant increase over the cost, complexity, and size of the circuit 100.
The problems identified above are in large part addressed by a flip-flop circuit according to the present invention in which a clock signal and a data input signal are received and a corresponding output produced. The circuit includes a set of series inverters connected to the clock signal to produce a delayed and complementary copy of the clock signal. The circuit further includes a set of three p-channel connectors connected in parallel between a supply voltage (Vdd) and a control node. The p-channel transistor gates are driven by the clock signal, the data signal, and the delayed signal. The circuit further includes three n-channel transistors connected in series between the control node and Vss and gated by clock signal C, data signal D, and the delayed signal. The control node controls the gate of a fourth p-channel transistor connected between Vdd and an output node. A set of three n-channel transistors is connected between the output node and ground. The gates of these transistors are controlled by the clock signal, the delayed signal, and an inverted copy of the data signal, which is provided directly to one of these output transistors via a control inverter. In one embodiment, the n-channel transistor string between the control node and ground and the n-channel transistor string between the output node and ground may share a common transistor having a W/L roughly twice that of the other n-channel devices. The output node may be connected to a stabilization circuit to improve noise immunity.